1. Field of the Invention
Broadly, the present invention relates to a self-actuating and self-locking flow cutoff valve. It particularly relates to use of such a valve in a nuclear reactor of the type which utilizes a plurality of fluid supported absorber elements to provide for the safe shutdown of the reactor.
2. Prior Art
There are numerous applications wherein there is a need for a self-actuating, self-locking flow cutoff valve. The need is particularly great in the case of nuclear reactors of the type which utilize a plurality of fluid supported neutron absorber elements to ensure the capability for a safe shutdown of the reactor. More particularly, heretofore nuclear reactors were typically shut down by control rods which were introduced through the top of the core and raised from or lowered into the core by mechanical means such as a motor which operates via clutch gears or the like. In an emergency, the clutch would be disengaged and the control rods allowed to fall into the core to shut down the reactor. Such a system had certain disadvantages. Specifically, there is a possibility that a mechanical device such as the clutch could not be disengaged or that some fault may have occurred which would distort the passage through which the control rods have to pass causing them to bind and preventing full insertion of the rods into the core. In such instance, it would not be possible to shut down the reactor. Accordingly, considerable interest has been generated in the use of a plurality of fluid supported neutron absorbing elements which would fall under the influence of gravity into the core in the event of a loss of fluid flow. Thus the reactor could be shut down by the simple expedient of shutting off the flow and further, in the event of an unforeseen loss of fluid flow, the reactor also would be shut down automatically.
U.S. Pat. No. 3,228,847 suggests a reactor control system which includes a control assembly for controlling neutronic flux. The control assembly comprises an inner tube extending from a nonactive region of the reactor into the active region, and an outer tube surrounding the inner tube and spaced therefrom. The outer tube has a closed end and the inner tube has an open end adjacent and spaced from the closed end of the outer tube. Neutron absorbing particles are positioned between the inner and outer tube for movement along the tube under the force of flow. The neutron absorbing particles are moved out of the active region of the reactor by fluid flow and fall back into the active region under the influence of gravity when the flow is shut off.
U.S. Pat. No. 3,257,286 suggests a ball-type control for a nuclear reactor. A number of elongated conduits are positioned in the nuclear reactor so that the first section of the conduit is located within the core and an adjoining second section is located exteriorly of the core. Each conduit holds a number of individual bodies, each of which contains a high neutron absorption cross-section material. The movement of the neutron absorber bodies within the conduits is achieved by providing a source of pressurized fluid available to each end of the conduit for selectively positioning the neutron absorber bodies within the first and second sections of the conduit. It is stated that a fission reactor can be started up, shut down, or reactivity controlled during reactor operations by varying the location of the absorber bodies.
U.S. Pat. No. 3,347,747 discloses a control organization and method for a nuclear reactor. The reactor is provided with a number of laterally spaced vertical passageways in the region of the core and distributed throughout the area thereof. The passageways include a lower portion which extends generally throughout the height of the core and an upper portion which extends above the core into the reactor vessel. Positioned within and confined in each passageway is a movable means which contains a poison and which is movable from a lower position within the region of the core to an upper position in the passageway, where it is generally above the core. The poison-containing means is moved by gravity to its lower position and is moved from its lower to its upper position by means of a fluid which is directed upward in the passageway.
In U.S. Pat. No. 4,076,583 there is disclosed another control method for a nuclear reactor which comprises a plurality of elongated conduits extending through and above the core of a reactor. A plurality of neutron absorber elements are located within the conduit, and during normal operation form a stacked bed in the portion of the conduit extending above the core. That section of the conduit in which the stacked bed is formed is provided with a fluid bypass means, it having been found that such bypass means ensures the capability of reliably maintaining all of the absorber elements in the stacked bed and out of the core during normal operations and further minimizing the pressure drop of fluid flowing through the stacked bed during normal operation.
While all of the foregoing suggested techniques appear to offer advantages over reliance solely on a control rod system, there is still room for improvement. More particularly, in all of these systems where gravity is relied upon to cause the absorber elements to move into the core, any residual fluid flow, even though it may be below the minimum for safe operation of the reactor, acts to retard the fall of the absorber elements. For example, in the event of a complete power failure, the inertia of a centrifugal pump would be sufficient to continue providing some flow after the loss of power and after the flow rate of fluid had dropped below the point at which the reactor should be shut down. Thus, clearly it would be advantageous to have a self-actuating flow cutoff valve in the fluid stream such that once the fluid flow dropped below a predetermined point, the valve would automatically close and substantially reduce the time required for the neutron absorbing elements to fall into the reactor core and safely shut it down. Further, in the event that there might be some erratic flow or surge of pressure or flow subsequent to it having declined below the safe level, such valve advantageously would be self-locking to prevent an inadvertent startup of the reactor by a resumption of fluid flow.